Imaging through turbid media has many diverse applications within aviation, defense, astrophysics, marine science, biology and medicine, etc. For example, imaging cars, ships and troops through fog, aircraft through clouds, body parts and foreign objects through human or animal blood and/or tissue or clothing, viewing through filled or fiber-reinforced plastics, or searching for objects in murky water are all applications where it may be desirable to improve visibility through turbid media. It is also often desirable to communicate through turbid media such as fog or seawater.
The turbid medium typically has at least two components, a first or base medium component having a first set of optical properties, and a second or scattering-particle component having a second set of optical properties. When photons pass through the turbid medium, they may pass through the base medium component between the scattering particles, and not be affected by the scattering particles; these photons are the ballistic photons. Alternatively, photons may pass through the base medium component until they are affected by a particle of the scattering particle component. These photons may then be deflected or reflected through a distribution of wide and narrow deflection angles by their interaction with the scattering particles, the distribution of deflection angles will vary according to the wavelength of the photons, the particle size of the particles, and optical properties of the base medium component and the scattering particle component. Photons may undergo multiple interactions with scattering particles, as well as an object of interest.
The mean free path Is represents the average distance a photon may travel in the medium before being scattered. The ballistic, or unscattered, photons have intensity I=Ioexp(−x/l*s) where Io is the input intensity, and x is the distance traveled. Snake photons are those that have been deflected by one or more scattering particles in a more-or-less forward direction, hence emerge from the media with small delay relative to the ballistic photons. Diffusive photons have been deflected many times or have been deflected through wide angles, hence emerging with longer delay and forming an incoherent background that can obscure objects of interest located within the medium.
Some turbid media, such as human and animal tissue, are considerably more complex than the above description, and may include a wide range of scattering particles and, at times it may be difficult to determine even what is base medium and what is scattering particles. Photons penetrating such media will, however, still become divided into populations of ballistic, snake, and diffusive photon components and, as the diffusive component becomes large relative to the ballistic component, vision through the media may be badly impaired.
Inhomogeneities in a medium, such as scattering particles, cause scattering which may alter the direction of propagation, polarization and phase of photons passing through the medium. The amount of light passing through the turbid medium may also be significantly reduced by absorption and scattering at particles within the medium. The net effect of the Inhomogeneities is to render many objects difficult to view without assistance.
When an object of interest, such as a road sign, car, vehicle, soldier, ship, bone, bullet, tumor, or other object is present within a turbid medium, the object of interest is illuminated by a combination of the ballistic, snake, and diffusive photons. These illumination photons interact with the object, and some of them being absorbed by the object and some of them are reflected by the object. Photons reflected by the object may in turn become divided into populations of ballistic, snake, and diffusive photons enroute to any imaging system observing the diffusive medium.
Prior systems for imaging in turbid media have attempted to separate diffusive from ballistic photons based upon the time of arrival of these photons at a sensor. Because transit time of ballistic photons must be known to set an acceptance window, these systems tend to work better with shadow imaging, requiring a sensor on an opposite side of the turbid medium from a light source; and do not work well with light reflected by an object of interest in the medium, where the transit time may not be known.